1. Field of the Invention
The present invention relates to a cable television (CATV) network, and more particularly, to an apparatus for performing a function required when a coaxial cable of a conventional hybrid-fiber coaxial (HFC) network is replaced with an optical cable in order to implement a fiber to the home (FTTH) network, and a data processing method used by the apparatus.
This work was supported by the IT R&D program of MIC/IITA.[2005-S-401-02, Optical Subscriber and Access Network Technology]
2. Description of the Related Art
Subscriber distribution networks can be classified into wired type networks and wireless type networks. Subscriber distribution networks use a wired/wireless integrated network to provide a service combining communication and broadcasting. Examples of currently widely used wired subscriber distribution networks include a digital subscriber line (xDSL) and a hybrid-fiber coaxial (HFC) network. Such subscriber distribution networks rapidly developed in the form of a fiber to the home (FTTH) network in response to consumer demand for an integration service in which broadcasting is combined with communication over an existing network mainly providing a multimedia service. In xDSL, a passive optical network (PON) is highly expected to be used in implementation of the FTTH network. In general, an Ethernet PON (EPON) or a gigabit PON (GPON) employing a time division multiplexing (TDM) method is used.
FIG. 1 illustrates a structure of a CATV network employing a conventional HFC network architecture.
Referring to FIG. 1, the CATV network includes a system operator (SO) 110, which is a service provider that provides a broadcast service to a subscriber by modifying a channel of a program received from a program provider (PP) and a channel of its own production program, and a cable subscriber distribution network 120 which transmits a signal from the SO 110 to a subscriber's cable modem 140. The cable subscriber distribution network 120 is referred to as a HFC network since an optical cable and a coaxial cable coexist from a distribution center 118 to the subscriber's cable modem 140. The cable subscriber distribution network 120 provides a CATV broadcast service and a data service. The SO 110 includes broadcast equipment 112, which sends received contents to a subscriber or sends the received contents to the subscriber after inserting its own advertisements or subtitles, a cable modem termination system (CMTS) 115, and the distribution center 118. The cable subscriber distribution network 120 of the CATV network includes an optical network unit (ONU) 122 which accesses an optical signal transmitted from the SO 110, i.e., the service provider, and a trunk line amplification splitter 125 which amplifies and splits a signal converted into an electronic signal by the ONU 122. In order to access the SO 110 using the optical signal and to transmit/receive a radio frequency (RF) signal to/from the subscriber's cable modem 140 using the coaxial cable, the ONU 122 performs an optical-to-electrical or electrical-to-optical conversion function. The trunk line amplification splitter 125 amplifies an attenuated signal so that the RF signal is transmitted to many subscribers. Alternatively, the trunk line amplification splitter 125 splits a signal so that connections can be made for many subscribers. The subscriber's cable modem 140 is a subscriber terminal device capable of interactive data communication. An RF signal channel is controlled by the CMTS 115.
FIG. 2 is a functional block diagram of an ONU in a CATV network employing a conventional HFC network architecture.
Referring to FIG. 2, in the ONU, a downstream transmitter 201 and an upstream transmitter 236 independently perform their functions. The downstream transmitter 201 includes an optical-to-electrical converter 210, a voltage regulator 215, equalizers 220 and 230, and a pre-amplifier 225 and a post-amplifier 235.
The optical-to-electrical converter 210 converts an optical signal transmitted from an SO into an electrical RF signal in the frequency range of 50 MHz to 780 MHz. The voltage regulator 215 regulates a voltage level of the RF signal. The first equalizer 220 equalizes a frequency characteristic of the RF signal whose voltage level is regulated. In particular, the signal equalizer 220 reproduces an attenuated high frequency component. Since a low power RF signal is received, the pre-amplifier 225 amplifies a signal level according to an input level required by the post-amplifier 235. In practice, a signal is split in the ONU so as to accommodate many subscribers. Therefore, the post-amplifier 235 is required. The second equalizer 230 equalizes a frequency characteristic of the amplified RF signal. Furthermore, the second equalizer 230 reproduces an attenuated portion of the signal after amplification. The post-amplifier 235 amplifies an actual RF signal by increasing it to a level suitable for transmission.
The upstream transmitter 236 includes an amplifier 250, an equalizer 255, a low-pass filter 260, and an electrical-to-optical converter 265.
The amplifier 250 amplifies the RF signal received upstream in the frequency range of 5 MHz to 48 MHz. The equalizer 255 equalizes a frequency characteristic of the amplified upstream RF signal. The low-pass filter 260 passes only a frequency band of the upstream RF signal so as to remove noise generated in a high frequency band. The electrical-to-optical converter 265 converts an electrical RF signal in the frequency range of 5 MHz to 48 MHz into an optical signal of 1310 nm and transmits the converted optical signal to the SO.
FIG. 3 is a functional block diagram of a cable modem in a CATV network employing a conventional HFC network architecture.
Referring to FIG. 3, the cable modem is a subscriber device used over a CATV network employing a conventional HFC network architecture for data communication and includes an RF tuner 310, a quadrature amplitude modulation (QAM) demodulator 320, a data over cable service interface specification (DOCSIS) media access control (MAC) module 330, a QAM modulator 340, and a controller 350.
The RF tuner 310 performs filtering in synchronization with a frequency channel band of 6 MHz allocated to the RF tuner 310 among received downstream signals in the frequency range of 50 MHz to 780 MHz. The QAM demodulator 320 performs QAM digital demodulation on the filtered downstream signal so as to restore an original digital signal. The DOCSIS MAC module 330 performs a core function of the cable modem and is used to determine an upstream/downstream transmission frequency band for data communication with the CMTS included in the SO and to control transmission of a data signal. The DOCSIS MAC module 330 performs a standardized function. The DOCSIS 2.0 standard has now been completed. The QAM modulator 340 modulates a digital signal into a QAM signal in order to improve transmission efficiency in a predetermined frequency range when a signal is transmitted upstream. The controller 350 controls the DOCSIS MAC module 330 and performs a control function for operational management of modems.
In the past, CATV networks employing the conventional HFC network architecture mainly aimed at providing a CATV broadcast service unilaterally. Since an interactive data service such as an Internet service, a video on demand (VOD) service, and a telephony service is provided by utilizing a portion of bandwidth over the existing network, an upstream bandwidth is constrained. In order to solve the problem, a method has been taken into account in which a cell size is reduced while enlarging a bandwidth to 3 GHz. Furthermore, a cable modem standard such as DOCSIS 3.0 is being developed. However, unlike the xDSL, an RF signal is used in the CATV network. Therefore, signals are not easily converted into the FTTH format.
In terms of a service, in order for a cable provider to be able to provide an interactive data service, network management needs to be carried out on a real time basis so as to ensure signal quality of the HFC network. However, network management cannot be easily carried out over the existing HFC network.